There are several attempts known of aqueous polymer compositions stabilized against degradation.
United States Patent Application Publication No. US 2006/0027364 discloses an aqueous, viscoelastic treating fluid gelled with a crosslinked guar or guar derivative. The viscosity of this treating fluid and the thermal stabilization is improved by addition of a glycol.
U.S. Pat. No. 5,103,913 discloses aqueous fracturing fluids with improved temperature stability. These fluids contain guar gelling agent and a soluble bicarbonate. As shown in the patent examples these fluids are applicable for operations in the temperature range between 175° C. and 210° C.
U.S. Pat. No. 4,801,389 discloses aqueous fracturing fluids with improved temperature stability. These fluids contain guar gelling agent, zirconium or hafnium crosslinking agent and a bicarbonate salt. These fluids are applicable for operations in the temperature range between 80 and 120° C.
U.S. Pat. No. 4,721,577 discloses stabilized fracturing fluids with improved temperature stability. These fluids contain guar gelling agent and thiol derivatives of selected heterocyclic compounds. These fluids are applicable for operations in the temperature range between 90 and 200° C.
U.S. Pat. No. 4,686,052 discloses gels derived from solvatable polysaccharides and crosslinked with titanium or zirconium compounds. These gels are stabilized against shear degradation at elevated temperature by addition of an alkanolamine.
Canadian Patent No. 2,467,791 discloses dry blended particulate compositions for well treating comprising a particulate hydratable polysaccharide, a particulate crosslinking agent, a particulate base and a particulate sodium, potassium or ammonium pyrophosphate or oxalate. As a thermal stabilizer sodium thiosulfate is proposed.
In the last years, commercially successful exploration of subterranean deposits has become more difficult because the subterranean deposits are prospected in a deeper stratum. For this reason, the temperature of the stratum tends to rise and, in many cases, it reaches 150° C. or more. At such high temperatures, the applied polymers deteriorate, for example by thermal, chemical or physical stress degradation. This leads to a reduction in rheological properties of the aqueous polymer composition and creates a serious obstacle in high temperature applications. Accordingly, the inherent functions of the aqueous polymer composition fluid can be satisfactorily obtained only if degradation (e.g., caused by decomposition) of its rheological and chemical properties can be prevented even at elevated temperatures.
U.S. Pat. No. 6,794,473 discloses an acrylic acid polymer and a production process therefore. For stabilizing the acrylic monomer against premature polymerization small amounts of phenothiazine are used.
GB-A-2,106,958 and EP-A-208,373 disclose crosslinked polymers for fracturing subterranean formations. But these documents do not disclose how to stabilize a polymer composition against thermal stabilization. In EP-A-208,373, a selected alkanolamine is used in combination with selected crosslinkers to maintain the resistance to shear degradation of the crosslinked fluid even under conditions of use at elevated temperatures.
It would be desirable if a composition and method could be devised to stabilize and enhance the performance of aqueous polymer compositions. The present invention is intended to overcome the above problems and an object of the present invention is to provide stabilized aqueous polymer compositions. It has been found that the above described object can be attained by adding phenothiazine compounds to the aqueous polymer compositions or to precursors of it.